"He's clearly, in our opinion, the brightest of the bunch," said VanderSloot, founder and chief executive of the health care products company Melaleuca. "He has a great understanding of what it takes to have a robust economy and he has a real firm grasp on international affairs."

VanderSloot said he plans to raise money for Rubio, but has not decided how much to give personally.

In 2012, VanderSloot and his company gave $1.1 million to Restore Our Future, a super political action committee that helped GOP nominee Mitt Romney. He also was one of Romney's national campaign finance co-chairs, raising millions of dollars in bundled donations for both the 2008 and 2012 campaigns.

VanderSloot said he and his wife gave $50,000 in June to Conservative Solutions, an outside group backing Rubio's campaign.

VanderSloot has contributed $2,700, the maximum allowed by law, to the presidential campaign of former technology executive Carly Fiorina. He said Fiorina ran a close second in his deliberations about who to endorse, but he concluded she lacks Rubio's charisma.

"Marco Rubio has a big edge on Carly Fiorina on the likeability factor," he said. "He is just so dang likable."

VanderSloot is the latest major GOP donor to sign on with Rubio, joining investor Paul Singer and hedge fund manager Cliff Asness.

VanderSloot said many big donors have been hesitant to make a public endorsement before a clear front-runner emerged.

But he said such a delay will only help candidates such as billionaire businessman Donald Trump and retired neurosurgeon Ben Carson — candidates he doesn't think can unite the country and win the White House.

"We're stepping out earlier, because it's a dangerous game to play if we don't," he said.